This invention relates in general to a method of and a device for controlling the burn-off self-cleaning function of a measuring resistor, such as a heating wire serving for measuring purposes in an internal combustion engine. In particular, this invention relates to a method of and a device for stopping the burning of the resistor under predetermined operational conditions of the engine.
In internal combustion engines used especially in motor vehicles, there is known an air flow sensor including a measuring resistance in the form of a heated wire which after each turning off of the engine is sparked with electric current in order to burn off all of the deposited polluting particles. This is achieved by unbalancing a bridge circuit in which the heated wire forms a branch. It is true that such a repeated burn-off leads to satisfactory results nevertheless the service life of heated wires is impaired.
It has also been found that in air flow meters equipped with heated wires which are subject to frequent burn off at relatively low temperatures, only organic polluting particles can be burned off, whereas the deposition of silicate glasses from ambient dust and the like which may cause gradual impairment of measuring results, is not prevented. On the other hand, however, such silicate glass particles when subject to high temperature from the heated wire may become melted away. To this end burn off processes are known which are dependent either on operational parameters of the engine and can be performed during different time periods.
A problem occurring generally in known burn-off processes of such hot wires is the fact that a catalytic burn-off of the same wire will result. The catalytic burn-off can occur under certain operational conditions of the engine, for example after the engine is shut down and its rotary speed drops below 2000 RPM or 300 RPM. In this case, fuel vapors due to pulsations of the air column in the intake manifold reach the hot wire sensor. As a consequence the cleaning burn-off process can initiate the so called catalytic burning of the fuel vapors on the hot wire. The burning fuel releases a considerable amount of additional heat resulting in over heating of the wire sensor above the temperate limit originally set by the increased electric current needed for the cleaning burn-off process. The over heated wire sensor starts acting as a catalyser and the catalytic burning can continue even if the heating electrical current is disconnected.
Such over heating of the wire damages the same due to evaporation, oxidation, melting and dimensional changes of wire material. In other words, the term "catalytic burning" denotes a detrimental overheating of the wire sensor because of burning of fuel vapors intruding in the intake manifold at certain rotary speeds of the engine.